1. Field of the Invention
The present invention relates to a heat treatment method for performing diffusion to semiconductor wafers and oxidization thereof by using source gas, and to a manufacturing method of a solar battery and a heat treatment device using the heat treatment method of semiconductor wafers.
2. Description of the Related Art
Heat treating furnaces for semiconductor wafers, having a cylindrical tube made of quartz as a process chamber and a heater arranged on an outer periphery of the tube are commonly used. For example, when heat diffusion treatment of diffusing desired impurities into wafers is to be performed, impurity diffusion can be caused on surfaces of semiconductor wafers placed in the heated quartz tube by continuously introducing source gas, which contains a dopant, into the tube. This method is called “open tube diffusion” and is used also in an impurity diffusion process for a solar battery having silicon wafers as cells.
Forms of the heat treating furnace are divided into a vertical type and a horizontal type by placement methods of a quartz tube and the vertical furnace is designed to rotate semiconductor wafers in the quartz tube to enhance uniformity of treatment. In the field of solar batteries, horizontal furnaces that excel in mass production are often used and large heat treating furnaces having a quartz tube with a total length of 1000 millimeters (mm) to 1500 millimeters are used. When semiconductor wafers are to be processed in the horizontal furnace, a treatment boat for supporting upright semiconductor wafers (a single semiconductor wafer or one assembly including a plurality of semiconductor wafers arranged in parallel to an extending direction of the tube) is used.
A supply method of source gas is a primary element that affects uniformity of heat treatment, and a current plate for controlling a gas flow or an injector for uniformly supplying gas into a quartz tube may be used to enhance uniformity in impurity diffusion within wafer surfaces. The injector is a gas introducing tube installed in a long quartz tube and has a plurality of gas injection holes to uniformly supply gas into the quartz tube.
In an example of the heat diffusion treatment where phosphorous (P) is to be diffused as n-type impurities into silicon (Si) semiconductor wafers, phosphorus trichloride (POCl3) is vaporized and mixed with nitrogen gas or oxygen gas to obtain source gas. Reaction formulae thereof are as follows:2POCl3+(3/2)O2→P2O5+3Cl2  (1)P2O5+(5/2)Si→2P+(5/2)SiO2  (2)
Chemical reactions shown in the formulae (1) and (2) occur in a furnace at 800° C. to 1000° C.
When oxidization treatment is to be performed, oxygen gas, water vapor, or the like may be used as source gas.
Japanese Utility Model Application Publication No. 63-098627 describes a method of uniformizing treatment by placing a quartz plate as a current plate for every predetermined number of semiconductor wafers on a boat made of quartz. Japanese Patent Application Laid-open No. 2009-194001 describes a heat treatment furnace in which four injectors (gas outlet pipes) are arranged in a tube and gas injection holes (openings) are arrayed according to a pitch in which semiconductor wafers are arranged. In both conventional techniques, the semiconductor wafers are arranged perpendicular to a tube extending direction, that is, in such a manner that vertical lines of planes constituting front or rear surfaces of the semiconductor wafers extend along the tube extending direction.
When the current plate or the injector is used as described above, in-plane uniformity in a semiconductor wafer or uniformity among individual semiconductor wafers can be enhanced. However, even when the method as described in Japanese Utility Model Application Publication No. 63-098627 or Japanese Patent Application Laid-open No. 2009-194001 is used, the source gas needs to be discharged from an end of the tube and thus a flow of the source gas in a direction of traversing the arranged semiconductor wafers occurs. The source gas has a substantially atmospheric pressure and the gas flow is a viscous flow. Accordingly, behavior of the gas near peripheries of the semiconductor wafers is complicated. For example, an air eddy may occur near the peripheries of the semiconductor wafers, or gas disturbance easily occurs in spaces between the arranged semiconductor wafers due to entry of the gas into the spaces between the semiconductor wafers. Such situations are factors that cause non-uniformity in the semiconductor wafer treatment.
Meanwhile, fluctuations in an amount of heat diffusion to the semiconductor wafers cause fluctuations in device characteristics. For example, in a case where impurities are to be heat-diffused into wafers for solar battery cells, sheet resistance is increased and conduction loss is increased when a diffusion amount is insufficient, while many defects arise in the wafers and photoelectric conversion efficiency is reduced due to recombination of carriers when a diffusion amount is too large.